CARBOCATION-LIKE REACTIVITY PATTERNS IN X'-SUBSTITUTED-4-BIPHENYLYLINITRENIUM IONS

4-Azido-X'-substituted biphenyls (X' = 4'-MeO; 4'-Me, 4'-F, 3'-Me, 4'- Cl, H, 3'-MeO, 3'-Cl, 4'-CF3) have been prepared and subjected to 248 nm flash photolysis irradiation in 20:80 acetonitrile:water. Transient X'-substituted 4-biphenylylnitrenium ions 10 (Ar-C6H4-N+H) are observ ed, with lifetimes ranging from 0.6 ms (4'-MeO) to 26 ns (4'-CF3). The se cations are quenched by azide ion, with values of k(az) ranging fro m 6 to 10 x 10(9) M-1 s(-1), with the majority in the range (9-10) x 1 0(9). This near constant k(az) provides further evidence that arylnitr enium ions are quenched by azide ion at the diffusion limit. The solve nt reactivities, plotted in a single-parameter Hammett plot against si gma(+)(X), exhibit a poor correlation, with the points for the para pi -electron donors deviating from the correlation line based on the othe r substituents in the direction of requiring a more negative substitue nt parameter. The data are more satisfactorily fit to the two-paramete r Yukawa-Tsuno equation; the parameter r(+) obtained in this fit is 2. 8. Thus the resonance interaction of the para pi-donor X'-substituents with the positive charge of the cation is underestimated by sigma(+), a situation that has previously been observed with benzylic-type carb enium ions. The conclusion is made that, in their reaction with water, 4-biphenylylnitrenium ions behave like benzyl cations bearing two add itional stabilizing Vinyl groups, i.e., as if they had the structure A r-C+(C=C)(2). The inherent reactivity and the pattern of the aryl subs tituent effects are in fact similar to those in the carbocation series Ar-C+(Ph)(2).